Field of Invention
The present invention relates to a wireless power transmitter circuit, particularly a wireless power transmitter circuit capable of operating in a constant current mode. The present invention also relates to a control circuit and a control method for use in the wireless power transmitter circuit.
Description of Related Art
It is desired for a wireless power transmitter circuit to be capable of operating in a constant current mode, providing a constant current. Conventional wireless power transmitter circuits need to operate according to a high input voltage, which leads to many problems. FIG. 1 shows a prior art wireless power transmitter circuit (wireless power transmitter circuit 1) which comprises a buck converter circuit, a power inverter circuit, a resonant transmitter circuit 11 and a resonant matching circuit 12, wherein the resonant current IRS is determined by the input voltage VS of the power inverter circuit.
The prior art circuit in FIG. 1 has drawbacks that it requires a relatively high level of input voltage, a relatively wide voltage control range and a digital to analog converter with relatively high resolution in order to meet the requirement for resonant current control. FIG. 2 shows characteristic curves of input voltage level versus corresponding reflected resistance when the wireless power transmitter circuit 1 operates in 16 W. As shown in the figure, a full-bridge class D power inverter requires an input voltage range of 0-33V, a class E power inverter requires an input voltage range of 0-42V, and a half-bridge class D power inverter requires an input voltage range of 0-66V. Besides, a digital to analog converter with a relatively high resolution is also required, especially when the reflected resistance is low.
Compared to the prior art in FIG. 1, the present invention is capable of achieving a constant current mode by analog control, with a smaller input voltage range or a fixed input voltage; the present invention has advantages such as simpler circuit architecture, faster response and lower cost.